1. Field of the Invention
The present invention relates to an optical amplifier, and more particularly to a laser amplifier for amplifying a laser propagating in an amplifier medium at a high gain as well as an optical fiber laser.
All of patents, patent applications, patent publications, scientific articles and the like, which will hereinafter be cited or identified in the present application, will, hereby, be incorporated by references in their entirety in order to describe more fully the state of the art, to which the present invention pertains.
2. Description of the Related Art
In recent years, a wavelength multiplexing transmission technique is important for an optical communication with a large capacity. An optical fiber comprising a quart-glass has widely been used as a medium for allowing a signal light propagation for the optical communication. A low loss wavelength band of the quart-glass fiber is ranged from 1450 nm to 1650 nm. In this wavelength band, C-band (1530 nm to 1560 nm-wavelength band) and L-band (1570 nm to 1610 nm-wavelength band) have been utilized for the optical communication, wherein those C-band and L-band may be amplified by an erbium doped fiber amplifier. In order to response to an increased total transmission capacity, it is effective to broaden the transmission wavelength band. In this point of view, another lower loss wavelength of 1650 nm-band is attractive.
The existent erbium doped fiber amplifier limits the amplification wavelength at about 1610 nm for the following reasons. The existent erbium doped fiber amplifier performs amplification depending upon a stimulated emission transition of erbium ions from the excited state absorption lower level (4I13/2) to the ground level (4I15/2). In the existent erbium doped fiber amplifier, a signal excited state absorption of a signal light with a wavelength of at least 1610 nm is caused by the erbium ion at the excited state absorption lower level (4I13/2), whereby a gain is reduced. This is unavoidable.
Whereas some optical amplifiers of the 1650 nm-wavelength-band have been reported, no practicable optical amplifier has been yet realized in view of efficiency, gain, fabrication of the fiber, and reliability thereof.
One of the conventional optical amplifier with the 1650 nm-wavelength band has been reported in 1996, IEEE, Photonics Technology Letters, Vol. 8–3, pp. 349–351, wherein a thulium ion (Tm3+) doped fluoride glass fiber amplifier is presented. Thulium ions doped in the cladding region of the fiber absorb a stimulated emission light with a peak wavelength of near 1.85 micrometers which is caused by a stimulated emission transition from an energy level (3F4) to an energy level of another energy level (3H6), so that a stimulated emission only remains in the 1650-nm-wavelength-band which corresponds to a short wavelength side base portion of this transition, whereby a gain of at least 25 dB could be obtained in the wavelength band from 1650 nm to 1670 nm. This technique is disclosed in Japanese laid-open patent publication No. 8-152531.
The optical amplifier disclosed in the above publication performs a low amplification efficiency of 0.22% in consideration of the non-optimum configuration thereof. The above publication is silent on the noise figure. In this conventional optical amplifier, an additional dopant of terbium ions is introduced into the cladding region and a majority of the emission energy is absorbed by terbium ions. For this reason, a light-to-light conversion efficiency is much lower than the conventional erbium doped fiber amplifier.
Another reported was made in 1990 IEEE Photonics Technology Letters, vol. 2–6, pp. 422–424, wherein a thulium ion (Tm3+) doped quartz-glass fiber performs a gain of 2 dB at a wavelength of 1.69 micrometers. This gain does not satisfy the actual requirement for a relay amplifier in the optical communication system.
A praseodymium-doped selenide glass fiber amplifier with the 1650 nm-wavelength-band is disclosed in European Conference On Optical Communication in 2000, but this fiber amplifier has not yet been practiced.
A broadband fiber Raman-amplifier over 1.65-micrometers-band employing high non-linear optical fiber is disclosed in Proceedings Of The 2000 Communications Society Conference If IEICE, but this fiber amplifier has not yet been practiced.
In 1990 IEEE Electronics Letters vol. 26–10, pp. 649–651, it is disclosed that an emission spectrum due to an emission transition from the laser upper level (2H11/2) to the laser lower level (4I9/2) or another emission transition from the laser upper level (4S3/2) to the laser lower level (4I9/2) is broaden in the wavelength band from 1620 nm to 1720 nm as shown in FIG. 16. The laser upper levels (4S3/2) and (2H11/2) are different in energy level by approximately 830 cm−1, and which are thermally coped to each other by an energy (210 cm−1) at an ordinarily temperature. The laser upper levels (4S3/2) and (2H11/2) will, hereinafter, be considered to be an united energy level (4S3/2, and 2H11/2).
A probability of a spontaneous emission transition from the laser upper level (4S3/2) to the ground level (4I15/2) of erbium ion is higher by 18 times than a probability of a spontaneous emission transition from the laser upper level (4S3/2) to the laser lower level (4I9/2) of erbium ion. This spontaneous emission light has a 550-wavelength-band. A probability of another spontaneous emission transition from the laser upper level (4S3/2) to the excited state absorption lower level (4I13/2) of erbium ion is higher by 7 times than the probability of the spontaneous emission transition from the laser upper level (4S3/2) to the laser lower level (4I9/2) of erbium ion. This spontaneous emission light has a 850-wavelength-band.
A laser emission at a 546 nm-wavelength due to the emission transition from the laser upper level (4S3/2) to the ground level (4I15/2) with a single wavelength excitation at 801 nm is disclosed in 1991 IEEE, Electronics Letters Vol. 27–20, pp. 1785–1786.
A laser amplifier at a 850 nm-wavelength due to the emission transition from the laser upper level (4S3/2) to the excited state absorption lower level (4I13/2) with a single wavelength excitation at 801 nm is disclosed in 1991 IEEE, Electronics Letters Vol. 27–2, pp. 184–186.
A laser emission at a wavelength of 2.7 micrometers due to an emission transition from an energy level (4I11/2) to the excited state absorption lower level (4I13/2) of erbium ion is disclosed in 1995 IEEE, Electronics Letters Vol. 31–5, pp. 373–374, wherein a secondary laser emission at a wavelength of 1.7 micrometers is caused at the same time of a primary laser emission of a wavelength of 2.7 micrometers for assisting the primary laser emission.
The existent erbium doped fiber amplifier limits an amplification wavelength band into a 1610 nm-band. The amplification of the signal light at the 1650 nm-band due to a stimulated emission transition from the laser upper level (4S3/2, and 2H11/2) to the laser lower level (4I9/2) of erbium ion. In this case, a life-time of erbium ion at the excited state absorption lower level (4I13/2) is long, for example, 10 ms. For this reason, the signal light excited state absorption reduces the gain.
About 50% of the accumulated energy at the laser upper level (4S3/2, and 2H11/2) are lost by an omnidirection emission including a spontaneous emission of a 550 nm-wavelength-band due to the emission transition from the laser upper level (4S3/2) to the ground level (4I15/2) and another spontaneous emission of a 850 nm-wavelength-band due to the emission transition from the laser upper level (4S3/2) to the excited state absorption lower level (4I13/2). This results in a reduced efficiency of the amplifier.
In the fiber amplifier, a part of the spontaneous emission light is amplified with a propagation thereof through the fiber amplifier. This amplified spontaneous emission light is so called to as amplified spontaneous emission light. The amplified spontaneous emission light prevents an optical amplification at a 1650 nm-wavelength-band due to the emission transition from the laser upper level (4S3/2, and 2H11/2) to the laser lower level (4I9/2), whereby any high gain can not be obtained.
In the above circumstances, the development of a novel optical amplifier free from the above problems is desirable.